Conventionally, there has been known an optical scanning observation apparatus which irradiates an object of observation with laser light and scans the irradiation position thereof, so as to convert into electric signals, using a photoelectric conversion means, signal lights including transmitted light, reflected light, and fluorescence light obtained from the object of observation, to thereby form image data. Examples of the apparatus may include: a laser scanning microscope using a galvanometer scanner as the scanning mechanism; and an optical scanning endoscope which irradiates an object of observation with laser light emitted from a fiber in such a manner as to form a spot on the object of observation, and oscillates the fiber so as to scan the laser light on the object of observation, to thereby acquire signal light to form an image.
In the scanning mechanism employed in the aforementioned optical scanning observation apparatuses, the scanning rate (scanning line rate) at the irradiation point on the object of observation may not necessarily stay constant, depending on the control method and the scanning pattern of the mechanism. For example, in a scanning mechanism which is oscillated at a resonance frequency in a uniaxial direction, the movement of the scanning mechanism in the oscillating direction is controlled according substantially to a sinusoidal function, and thus the scanning rate on the object of observation does not stay constant. Meanwhile, spiral scanning of an object of observation is characterized in that the scanning rate becomes higher with increasing distance from the scanning center or drawing closer to the periphery of the screen.
In general, in optical disks including CD and DVD, the disk rotation speed is adjusted depending on the distance from the disk center so as to keep the scanning rate constant, to thereby ensure the uniform recording density. However, in the case of a laser scanning microscope or an optical scanning endoscope where the scanning mechanism is operated at high speed using a resonance frequency, it often involves difficulty to maintain a constant scanning rate by adjusting the frequency depending on the scanning position.
When the scanning rate varies, brightness within the scanning range becomes non-uniform unless the rest of the conditions is changed. In light thereof, there has been proposed a method of adjusting the power of laser light so as to make it uniform the irradiation density within the scanning range, to thereby reduce non-uniformity in brightness (see, for example, Patent Literature 1).
Alternatively, in a case where the scanning rate varies while the sampling frequency is kept constant, the number of sampling points per unit area is high in a region with a low scanning rate, which means that the scanning is wastefully performed. In contrast, in a region with a high scanning rate, the number of sampling points per unit area is so small, which may cause inconvenience where no sampling point can be found within a pixel. In light thereof, there has been disclosed a method of keeping substantially constant the sampling density within the scanning range, to thereby avoid such problems (see, for example, Patent Literature 2).